This invention relates, in general, to the preparation of arsenic-selenium photoreceptors and, in particular, to fabrication of said photoreceptors wherein the arsenic distribution, particularly at its top surface, is controlled within a preferred concentration range.
The concept of xerography was originally described by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the field. The discovery of the photoconductive insulating properties of highly purified vitreous selenium has resulted in this material becoming a standard in reuseable commercial xerography. The outstanding advantages of vitreous selenium is its capability of holding and retaining an electrostatic charge for long periods of time when not exposed to light, and its relative sensitivity to light as compared to many other photoconductive materials. In addition, vitreous selenium exhibits excellent physical strength and stability to be reused or cycled thousands of times.
Vitreous selenium, however, does suffer from one serious disadvantage in that it becomes unstable at temperatures slightly above about 100.degree. F. and begins to crystallize and become conductive in the dark rendering it unsuitable for use in xerography. U.S. Pat. Nos. 2,803,542 and 2,822,300 to Ullrich and Mayer et al, respectively, teach that the incorporation of elemental arsenic with selenium not only increases the spectral response of the selenium but in addition greatly increases the resistance of selenium to crystallization at elevated temperatures. In addition to alloying with arsenic, the addition of a halogen such as iodine or chlorine when added to arsenic-selenium alloys has been found to improve electrical characteristics such as sensitivity and spectral response. This contribution to the art is set forth in U.S. Pat. No. 3,312,548 to Staughan.
The arsenic-selenium alloys described in the above patents, are normally prepared by mixing a master alloy having the appropriate proportion of arsenic and selenium and placing the material in a closed container capable of being evacuated. The evaporation is carried out under vacuum conditions by heating a crucible containing the alloy mixture and allowing vapors of the arsenic-selenium alloy to condense and form a vitreous layer on a substrate normally supported above a crucible containing the alloy.
U.S. Pat. No. 3,911,162 to Erhart et al discloses a method of vapor depositing a uniform film of photoconductive material on a substrate which comprises positioning a plurality of substrate bodies on a plurality of elongated horizontal extending cylindrical mandrels, rotating each of said mandrels about an associated longitudinal axis thereof while simultaneously transporting the mandrels in an annular path about a horizontal axis. Vapor deposition is carried out under vacuum using a photoconductive material, such as an arsenic-selenium alloy, which is positioned in a planar array of crucibles which is located within the annular path of travel of the mandrels. The disclosure contained in U.S. Pat. No. 3,911,162 is incorporated herein by reference.
Employing the evaporation methods and equipment disclosed in U.S. Pat. No. 3,911,162 with selenium alloys containing up to about 0.5% by weight of arsenic fractionate, there is produced a progressive enrichment of arsenic content in the melt remainder. For the usual conditions in which a fixed charge of alloy is completely exhausted from the crucible, the photoreceptor also progressively enriches in arsenic level with the major portion of the arsenic content being concentrated in the final layers of the deposit, corresponding to the arsenic composition of the terminal melt volume. Generally, top surface arsenic contents of less than about 1% by weight or greater than about 5% by weight are considered undesirable. For many photoreceptors, including the Xerox 9200 and 3100 type photoreceptors, the preferred mean top surface arsenic content is about 2.5% by weight. Low top surface arsenic values may lead to photoreceptor failure through surface crystallization while high top surface arsenic levels can impair electrical properties such as residual voltage. For many photoreceptors including the Xerox 9200 and 3100 photoreceptors, the preferred mean top surface arsenic level is about 2.5% by weight of arsenic.